Conventional cellular mobile radio telephone systems include a land system and many mobile stations. The land system consists of at least one mobile switching center and several radio base stations which are distributed over a geographical area. Each radio base station serves a cell, i.e. a part of the geographical area, by sending and receiving information to and from mobile stations over radio channels. The mobile switching center, connected on one side to the public switching telephone network and on the other to the base stations, performs most of the control functions required for calls to and from the mobile stations in the geographical area. Because the cells are relatively small, mobile stations often travel between a number of different cells. The process of switching the established call from a home base station in one cell to a target base station in another cell is known as handoff.
In existing analog Frequency Division Multiple Access (FDMA) cellular systems, a base station determines the need for a handoff of mobile stations it is handling based on periodic measurements of the signal strength and signal quality of the uplink voice channel signals received from each mobile station engaged in a call. If the measured signal strength or signal quality is below a predetermined level, that home base station sends a handoff request to the mobile switching center. The mobile switching center queries neighboring base stations for reports of the previously measured signal strength of signals on the voice channel currently being used by the mobile station. The mobile switching center selects the neighbor base station reporting the strongest signal, provided that signal is above a threshold, and transmits appropriate commands to that target base station and to the mobile station via the home base station to tune to a specified new voice channel.
In dual-mode systems, as specified in the EIA/TIA IS-54 standard, handoff may also be implemented using Mobile-Assisted Handoff (MAHO) Using this procedure, a mobile station may be ordered by the land system to measure and report signal strength and other parameters of the digital radio channel emitted by the home base station, and the neighbor base stations This enables handoff requests by a home base station to be based not only on the signal strength and other parameters of the uplink signal received from the mobile station, but also on the down link signal parameters detected by the mobile station of the home and neighbor base stations. However, when performing a handoff from an analog to a digital channel, MAHO cannot be used because the mobile station cannot report signal strength or other signal parameter measurements over an analog channel.
In purely digital cellular systems, mobile-assisted handoff may always be used. For a more detailed U.S. patent application Ser. No. 371,126, filed Jun. 26, 1989.
There are a number of disadvantages associated with the presently known handoff methods described above. For example, if an analog voice channel is used in a call, a mobile station may cross several cell boundaries before its signal strength becomes too weak. Conventional handoff (without MAHO) does not account for the possibility that a geographically closer base station would provide a stronger, higher quality signal connection with that mobile station. Similarly, in cellular systems having a large umbrella cell that includes a number of small microcells, traditional handoff (without MAHO) requires a permanent and inflexible request procedure to handoff calls from the umbrella cell to the microcells because the signal strength of a mobile station travelling in the umbrella cell may never fall below the threshold under which handoff is initiated. Such a permanent handoff request procedure results in excessive signal processing.
Disadvantages are also associated with handoff in digital systems. In a purely digital system using Mobile-Assisted Handoff (MAHO), if a mobile station is moved around a street corner so that its signals cannot be received by its home base station, no more measurements can be reported by that mobile station. After a time period during which it is hoped to reestablish contact with the mobile station expires, handoff becomes uncertain because the signal measurements are several seconds old. Regarding how a handoff may be improved in this situation, reference is made to U.S. patent application Ser. No. 604,522, filed Oct. 25, 1990, which may be used together with the present invention and is incorporated herein by reference.